This invention relates to forming integral rivet joints, particularly as used in the attachment of operating tabs to metal self-opening can ends. The basic form of integral rivet construction for self-opening can ends, which has been commercially quite successful for the past thirty years, was the basis for a world-wide change in the can packaging industry. At present billions of metal cans are used for beverages, foods, and other materials, all featuring some form of self-opening construction. This seemingly simple configuration has, in fact, many complexities which are not apparent to the casual viewer.
Self-opening or "easy open" can ends basically consist of two parts. These are (1) the shell, which is the major element and (in cylindrical cans) is a disc-like member have a pre-formed perimeter which will later be attached to a full can body, (2) the tab, which is the operating part during the self-opening procedure, and (3) the integral rivet structure which joins the tab to the shell. The completed joined shell and tab constitute a self-opening end. A score on the shell defines an opening panel which is at least partially separated from the shell material during opening action of the tab. Many beverage cans now employ a retained tab, which remains attached to the end after the opening action.
Basically, the integral rivet is formed of an area, usually referred to as a bubble, raised from the plane of the shell material and then shaped into a rivet button, to fit closely within a hole in the operating tab. After the tab is placed around the button, and set flat against the exterior (public side) of the end, the top of the button, passed through the hole on the tab, is staked, i.e. forced down onto the tab, to complete an integral rivet, one in which the integrity of the metal of the end is not violated in any way. In that fashion, the tab is attached to the end while the end remains a single unpierced piece of metal, and the end is later attached to the open top of a filled can by known means.
The ends must withstand both internal and external pressures, must not interact unfavorably with the can contents, must at all costs not rupture until opened, and must function efficiently that one time, when the user is ready to open the can, even though it may have had a shelf or storage life of many months. As usage of this type of can package increases, more attention has been given to the economies of metal usage; thinner metal, and different types of metal, are introduced, and these factors in turn affect the ability of the tooling to operate effectively on these different types of metals and still produce, at high speed over long periods of operation, ends which will not rupture and which will perform their one-time opening function when brought into play.
By way of example, the need for adequate buckle strength dictates the types of materials which may be used for making can ends. As pointed out, the trend is to thinner, harder materials, with coatings that have lubricants incorporated in them rather than applied to them. These materials must run properly over tooling systems, but those same systems must be able to work with older materials also. The differences in strength, and in coatings, between such materials create a need for a new approach to tooling design which makes the tooling relatively insensitive to material changes and still able to form acceptable integral rivet joints at the higher operating speeds which now prevail.
Thus, the varieties of metal choice, coatings and end and tab design all combine to present a complex situation to the tool designer. The tooling is typically operated in a reciprocating press, which may be single or double acting, to perform a sequence of progressive operations on the shell, and to attach the tab. A disclosure of one currently operating press/tooling conversion system is found in U.S. Pat. No. Re. 33,061 granted Sep. 19, 1989 to the assignee of this application. The embodiment shown in that patent has two lanes of tooling stations and produces two ends simultaneously, however, newer version of that system utilize three lanes, and operate at speeds in the order of 600 strokes/min. Thus, the tooling must operate rapidly, very accurately, and over long operating periods. It is common to run such conversion presses 22 hours/day, allowing 2 hours/day for maintenance or repair.
Considerable attention has been given to methods and tooling for the above-described operations. Tooling is designed to define the area of the end from which the bubble is formed, and to cause the metal of that area to flow in certain ways. Different specific processes, and tooling to carry out such processes, have been used over the past years to accomplish this purpose. Such prior processes can be generally characterized as including one or more steps of drawing material from the end and reshaping (usually further drawing) the metal into the rivet button. It has been discovered, however, that to achieve a process and tooling which is essentially insensitive to variations in material, both as to thickness and flow characteristics, it is desirable to minimize drawing of the metal.
It is necessary also to address the tab itself, and the region of the end surrounding the button and from which the button is integrally formed. The trend toward thinner materials has a direct and profound effect on the region of the tab surrounding the hole through which the rivet button is projected. The basic rule is, the thinner the tab material, the greater the area of rivet head needed over the tab to prevent tear out of the tab from the rivet when the tab is actuated, usually by lifting. Need for more material in the finally formed rivet head in turn affects the amount of material, and the uniformity of wall thickness, in the button.
Practically all can ends are formed of coated metal of some kind, usually either aluminum or steel. Typical aluminum materials which have been used are 5000 Series metals, with type 5182 H19 being the predominant choice. Some users have sought to use 3000 Series aluminum, which is widely used for aluminum can bodies. This metal has lower yield and tensile strengths, and has been noticed to be more abrasive to tooling as compared to the 5182 aluminum. Similar situations are found with steel sheet. The more commonly used is T-5 (temper 5) steel, but DR-9 (double reduced) steel is being introduced to this market since it has higher yield and tensile values, but it is more difficult to form.
In the U.S. most coatings are added at the mill (aluminum or steel), and the coated materials are available from the supplier with allowances already incorporated in their specifications. On the other hand, in many foreign countries coatings are applied to metal stock sheet by a third party, or by the can and end manufacturer. Coatings (applied to both sides of the metal sheet), and particularly their processes of application, can make substantial changes in the strength and workability of the basis metal to which the coatings are applied, due primarily to the heat used and the period of time to which the metal is exposed to such heating. Lubricants are added to the coatings, with the trend toward included lubricants which are a part of the coating itself, rather than simply applied to the coating exterior. One reason for this is that externally applied waxes will interfere with printing on the public side of the ends.
The consideration of importance here is that the coating on the metal, however it is created, and regardless of its nature and uniformity, must not be violated during the operation of the tooling on the materials. Metal exposure to can contents can lead to undesirable reactions between the contents and the exposed metal, e.g. beer vs. uncovered steel, or carbonated beverages or certain food products vs. aluminum.
As mentioned, varieties of metal choice, coatings and end and tab design all combine to present a complex situation to the tool designer.